In the case of a wavelength division multiplexing (WDM) transmission system, transmission capacity may be effectively enhanced by increasing a transmission rate, reducing a channel spacing or widening a transmission wavelength range.
Recently, the transmission rate of the system has increased from 2.5 Gb/s to 10 Gb/s, and the transmission system having 40 Gb/s will be generally used in the near future. A power per channel is increased so as to enhance the transmission rate, but noises and non-linearity in optical fibers are increased and then a transmission property is deteriorated if the power per channel is increased as described above.
In the system which is transmitted in a long distance at a transmission rate of 40 Gb/s, the noses may be lowered by using a Raman amplifier instead of a conventional EDFA (Erbium Doped Fiber Amplifier). Also, because tolerance of a signal distortion system by dispersion is proportionate to an inverse square of the transmission rate, the tolerance is decreased to a level of 1/16 at a receiving end if the transmission rate is increased 4 times. Accordingly, precise dispersion compensation is required so that cumulative dispersion of the transmission channel cannot exceed the tolerance in the system with a transmission rate of 40 Gb/s. For this purpose, RDS (Relative Dispersion Slope) of a dispersion-compensation optical fiber should be similar to RDS of an optical fiber used as a transmission line (wherein, RDS is a value obtained by dividing a dispersion slope by the dispersion).
Also, in order to enhance the transmission capacity, a channel spacing of the system has been narrowed from 200 GHz (1.6 nm) and 100 GHz (0.8 nm) to 50 GHz (0.4 nm) and 25 GHz (0.2 nm) or less. However, as the channel spacing becomes gradually narrowed, signal distortion is caused by a four-wave-mixing phenomenon, or a non-linear phenomenon such as cross phase modulation and XPM (Cross Phase Modulation). Especially, if the low dispersion of the optical fiber is nearly close to the phase-matching condition, cross talk power is caused by a four wave mixing, finally causing a signal distortion.
Intensity of the cross talk power is associated with a power per channel, a channel spacing of the system, and dispersion and effective sectional area of the optical fiber. If the power per channel is reduced to decrease intensity of the cross talk power, an optical signal noise ratio becomes worse, and therefore transmission distance gets short, resulting in an increased cost of the system upon long-distance transmission.
Also, intensity of the cross talk power is lowered as dispersion of the optical fiber increases, but its loss is increased since a length of the used optical fiber for dispersion compensation gets longer in proportion to the dispersion of the optical fiber. Accordingly, the dispersion of the optical fiber should be optimized depending on properties of the system.
Also, an effective sectional area of the optical fiber, which is referred to as light intensity per unit area, is useful to inhibit non-linear phenomenon as the effective sectional area is greater.
As describe above, in addition to accelerate a transmission speed or narrow a channel gap of a transmission wavelength, it is possible to increase transmission capacity by widening an amplifiable wavelength range. In this case, it is required to widen a transmission wavelength range to conventional C band (1,530˜1,565 nm) and L band (1,565˜1,625 nm), as well as O band (1,260˜1,360 nm), E band (1,360˜1,460 nm) and S band (1,460˜1,530 nm). In this case, it is, however, impossible to use the EDFA (Erbium Doped Fiber Amplifier) to amply signals positioned at the O-E-S bands.
A Raman amplifier rather than EDFA should be used for amplifying signals of a short wavelength range corresponding to the O-E-S bands. Raman amplification, which uses SRS (Stimulated Raman Scattering) phenomenon referred to as a non-linear phenomenon generated in the optical fiber, amplifies signal intensity by converting a power of a pump wavelength into a transmission signal wavelength.
That is to say, if a zero-dispersion wavelength of the optical fiber exists at a pump signal wavelength range, new signals are generated at the transmission signal wavelength range due to crosstalk between the pump signals, resulting in increased noises. Accordingly, the zero-dispersion wavelength of the optical fiber should get out of a range of the pump signal wavelength if the Raman amplification is used. Also, the optical fiber should have a low effective sectional area and a reduced loss so as to improve Raman gain efficiency.
U.S. Pat. No. 5,835,655 discloses an optical fiber in which a zero-dispersion wavelength is shifted to escape from a transmission wavelength range, and an effective sectional area is increased to at least 70 μm2 to prevent a non-linear phenomenon. The optical fiber of the U.S. Pat. No. 5,835,655 may prevent the non-linear phenomenon because it has the effective sectional area of at least 70 μm2, and may inhibit a signal distortion by a four-wave mixing in the C-band because a zero-dispersion wavelength is located in a wavelength range of 1,500˜1,540 nm or 1,560˜1,600 nm. However, described was not the patent the optical fiber whose zero-dispersion wavelength exists on 1,300 nm or less to use O-E-S bands as the transmission line, and effective sectional area is suitably adjusted to inhibit the non-linear phenomenon and also enable the Raman amplification.
Also, U.S. Pat. No. 6,396,987 discloses an optical fiber capable of reducing a cost for the dispersion compensation, compared to general single mode optical fibers in a system having a transmission rate of 40 Gb/s. That is to say, the optical fiber of the U.S. Pat. No. 6,396,987 has dispersion of 6˜10 ps/nm-km, a dispersion slope of 0.07 ps/nm2-km or less, and an effective sectional area of at least 60 μm2 at a 1,550 nm. In this case, the optical fiber has a problem that the signal distortion by the four-wave mixing arises in a pump wavelength range because the zero-dispersion wavelength is located near 1,460 nm, for example near a pump wavelength for Raman amplification.